Look for this MU, either in Univiewer Console by displaying tasks and session and filtering on this MU, Or by the command line

$UNI_DIR_EXEC/uxshw tsk tsk=* mu="MISSING_MU"

or

%UNI_DIR_EXEC%\uxshw tsk tsk=* mu="MISSING MU"

Delete the retrieved task or tasks if there are several one, or modify their MU and map it to a valid MU.

Additional Information:

Please note that this suggestion is a very probable reason for the problem but the issue can also be related to a "Session" where the "Execution Context" points to a missing MU or a Dependency pointing to a missing MU inside a Uproc.Commands like: "uxshw upr upr=*" or "uxshw ses ses=* lnk" should indicate what is the guilty object.